What are Transmissible Spongiform Encephalopathies?

Transmissible spongiform encephalopathies (TSEs) are a group of fatal degenerative diseases that affect the central nervous system and can occur in humans and certain animal species. They are characterized by microscopic vacuoles and the deposition of amyloid (prion) protein in the gray matter of the brain which causes it to appear "spongy". The most common TSE among humans is Creutzfeld-Jacob-Disease (CJD), which has a worldwide rate of approximately one case per million people each year.

Bovine spongiform encephalopathy (BSE), commonly referred to as "mad cow disease" is a TSE that affects cattle. It is thought to be due to the feeding of rendered bovine meat and bone meal to young calves.

Who are at risk for getting TSE?

TSE's are not known to spread by direct person-to-person contact. Human TSEs can occur in three different ways:

Sporadic-unknown origin, accounts for 85-90% of cases.

Familial- inherited in families- accounts for 5-10% of all cases.

Acquired or iatrogenic- caused by the accidental transmission of the causative agent during invasive medical interventions such as exposure to human cadaver-derived pituitary hormones, dural and cornea grafts, and contamination instruments used in neurosurgery. This accounts for less than 1% of all cases.

What are the symptoms of Transmissible Spongiform Encephalopathies?

What causes TSE?

The agent causing TSE is not known- it is not a bacteria, virus, parasite or any other infectious agent known to cause disease. It is known that infected animals and humans have abnormal proteins called prions which can be detected using various laboratory techniques.

How are Transmissible Spongiform Encephalopathies diagnosed?

There are several tests that can be used to aid in the diagnosis of TSE. Specialized laboratories can analyze cerebrospinal fluid (CSF), blood and brain tissue obtained at biopsy or at autopsy.

What is the treatment for TSE?

TSEs are invariably fatal and there is no treatment or prophylaxis.

How is BSE connected to TSE?

Eating beef from cattle infected with mad cow disease (BSE) is thought to trigger a fatal human variant of the illness- known as variant Creutzfeld-Jacob Disease (vCJD). vCJD was first reported in the United Kingdom in 1994. Until April 2002, there were no cases of BSE or vCJD in the United States; however there were concerns that cases could occur among United States residents with past exposure to contaminated meat overseas. In April 2002, the Centers for Disease Control and Prevention reported a likely case of vCJD in a British citizen residing in the United States. The patient's clinical condition and travel history are consistent with vCJD acquired in the United Kingdom. There have been no cases of vCJD in the United States due to contamination of US meat products.

How soon after infection do symptoms appear?

Symptoms of CJD may appear months or even years after exposure to the infecting agent. Since vCJD is such a new disease, it not known how long the incubation period is but it may be as long as decades after consuming a BSE-contaminated product. Britain stopped the practice of ruminant feeding in 1988, yet cases of vCJD in that country are still being diagnosed, confirming the long delay between infection and the appearance of symptoms.

How is classic CJD different from vCJD?

Most victims of classic CJD die in their late sixties after developing relatively slow-onset mental deterioration. In vCJD, most deaths have occurred among young adults in their late twenties and are often preceded by sudden behavior changes initially diagnosed as psychiatric illness.

Can we expect to see cases of Mad Cow Disease and vCJD in the United States?

In December 2003, a cow slaughtered on a farm near Yakima, Washington was confirmed to have a positive test for BSE. This was the first time that an infected animal has been detected in the United States since May 1990, when active surveillance and testing began. This cow was a "downer" (non-ambulatory at slaughter) and was included in the United States Department of Agriculture (USDA's) targeted BSE surveillance program.

Starting in 1989, the USDA began taking steps to prevent BSE from entering the United States, including severe restrictions on the importation of live ruminants, such as cattle, sheep and goats, and certain ruminant products from countries where BSE was known to exist. These restrictions were later extended to include importation of ruminants and certain ruminant products from all European countries.

The USDA has tested over 20,000 cattle annually in 2002 and 2003 for BSE using a targeted surveillance approach designed to test high-risk animals for BSE. This included downer animals, animals that die on the farm, older animals and animals exhibiting signs of neurological distress. Overall, the USDA has tested over 57,000 cattle since 1990. In addition, a feed ban has been in place since 1997, which forbade the practice of feeding rendered bovine meat or bone meal to US livestock. The Harvard Center for Risk Assessment concluded that this ruminant feed rule provides a major defense against BSE and vCJD in the US.

The USDA is currently reevaluating its BSE screening program and considering, both increasing the number of cattle tested as well as looking into more rapid tests such as are currently being used in Europe. It is estimated that about 195,000 cattle are non-ambulatory or sick at slaughter and therefore eligible for testing. On December 30, 2003 the USDA announced additional safeguards to further minimize the risk of human exposure to BSE in the US.

The USDA has stated that this isolated bovine case does not indicate any increased risk to the American beef supply.